Physical Layer Issues and Methods Outline 802.3 Physical Layer Ethernet Technology Physical Layer Encoding Final Exam Review - ?? Ethernet Standard Defines Physical Layer • 802.3 standard defines both MAC and physical layer details • Even though we have worked from top down, Ethernet was about hardware first Metcalfe’s original Ethernet Sketch Fall, 2001 CS 640 2 Physical Layer Configurations for 802.3 • Physical layer configurations are specified in three parts • Data rate (10, 100, 1,000) – 10, 100, 1,000Mbps • Signaling method (base, broad) – Baseband • Digital signaling – Broadband • Analog signaling • Cabling (2, 5, T, F, S, L) – – – – Fall, 2001 5 - Thick coax (original Ethernet cabling) F – Optical fiber S – Short wave laser over multimode fiber L – Long wave laser over single mode fiber CS 640 3 Ethernet Technologies: 10Base2 • 10: 10Mbps; 2: under 185 (~200) meters cable length • Thin coaxial cable in a bus topology • Repeaters used to connect multiple segments – Repeater repeats bits it hears on one interface to its other interfaces: physical layer device only! Fall, 2001 CS 640 4 10BaseT and 100BaseT • 10/100 Mbps rate • T stands for Twisted Pair • Hub(s) connected by twisted pair facilitate “star topology” – Distance of any node to hub must be < 100M Fall, 2001 CS 640 5 Switched Ethernet • Switches forward and filter frames based on LAN addresses – It’s not a bus or a router (although simple forwarding tables are maintained) • Very scalable – Options for many interfaces – Full duplex operation (send/receive frames simultaneously) • Connect two or more “segments” by copying data frames between them – Switches only copy data when needed • key difference from repeaters • Higher link bandwidth – Collisions are completely avoided • Much greater aggregate bandwidth – Separate segments can send at once Fall, 2001 CS 640 6 Physical Layer Data Transfer • Signals are placed on wire via transceivers • Problem is how to do transmit 0’s and 1’s (signal encoding) in a robust fashion – Binary voltage encoding • Map 1 to high voltage • Map 0 to low voltage – How are consecutive 0’s or 1’s detected at node? • Clock synchronization problem • Transmitted signals have a variety of problems – Attenuation – Noise – Dispersion Fall, 2001 CS 640 7 Encoding Taxonomy • Digital data, digital signal – Codes which represent bits – Our focus – Many options! • Analog data, digital signal – Sampling to represent voltages • Digital data, analog signal – Modulation to represent bits • Analog data, analog signal – Modulation to represent voltages Fall, 2001 CS 640 8 Encoding Requirements • Small bandwidth – • Low DC level – • Increases transmission distance Frequent changes in the voltage – • Enables more efficient use of signaling capability Enables synchronization between the transmitter and the receiver without the addition of extra signal Non-polarized signal – Enables use of 2-wire cable to not be affected by the physical connection of the wires. Fall, 2001 CS 640 9 Non-Return to Zero (NRZ) • High voltage = 1 and low voltage = 0 • Voltage does not return to 0 between bits • Receiver keeps average of signal seen to distinguish 0 from 1 Fall, 2001 CS 640 10 NRZ • Benefits – Easy to engineer – most basic encoding – Efficient use of bandwidth – not many transitions • Drawbacks – – – – Fall, 2001 Long strings of 0’s can be confused with no signal Long strings of 1’s can cause signal average to wander Clock synchronization can be poor High DC – average of ½V CS 640 11 NRZ-Inverted (NRZI) • NRZI addresses clock synchronization problem – Encodes 1 by transitioning from current signal – Encodes 0 by staying at current signal • So we’re still out of luck on consecutive strings of 0’s Fall, 2001 CS 640 12 Manchester Data Encoding • Explicit merging of clock and bit stream – Each bit contains a transition • High-low = 1 • Low-high = 0 – Enables effective clock signal recovery at receiver • Clocks are still needed to differentiate between bit boundaries • Poor bandwidth utilization – Effective sending rate is cut in half • Used by 802.3 – 10Mbps Ethernet Fall, 2001 CS 640 13 Manchester Encoding contd. 0 0 +V +V -V -V Encoding for 0 Encoding for 1 0 1 0 1 1 1 0 +V -V Bit Boundaries Fall, 2001 CS 640 Signal Edges 14 4B/5B Encoding • Tries to address inefficiencies in Manchester • Idea is to insert extra bits in bit stream to break up long sequences of 0’s or 1’s • Every 4 bits of data are encoded in a 5 bit code – Encodings selections • At most one leading 0 • At most two trailing 0’s • Never more than three consecutive 0’s • Uses NRZI to put bits on the wire • This is why code is focused on zeros • 80% efficiency • See text for details of codes Fall, 2001 CS 640 15